
CRT Controller Registers
9-5
`efmp69030 Databook
Revision 1.3 11/24/99
CR03
Horizontal Blanking End Register
read/write at I/O address 3B5h/3D5h with index at address 3B4h/3D4h set to 03h
shadowed for pipelines A and B
7
Reserved
Values written to this bit are ignored. To maintain consistency with the VGA standard, a
value of 1 is returned whenever this bit is read. At one time, this bit was used to enable
access to certain light pen registers. At that time, setting this bit to 0 provided this access,
but setting this bit to 1 was necessary for normal operation.
6-5
Display Enable Skew Control
Defines the degree to which the start and end of the active display area are delayed along
the length of a scanline to compensate for internal pipeline delays.
These 2 bits describe the delay in terms of a number character clocks.
4-0
Horizontal Blanking End Bits 4-0
These 5 bits provide the 5 least significant bits of either a 6-bit or 8-bit value that specifies
the end of the blanking period relative to its beginning on a single scanline.
In standard VGA modes, where bit 0 of the I/O Control Register (XR09) is set to 0, the
horizontal blanking end is specified with a 6-bit value. The 5 least significant bits of the
horizontal blanking end are supplied by these 5 bits of this register, and the most significant
bits is supplied by bit 7 of the Horizontal Sync End Register (CR05).
In extended modes, where bit 0 of the I/O Control Register (XR09) is set to 1, the horizontal
blanking end is specified with an 8-bit value. The 5 least significant bits of the horizontal
blanking end are supplied by these 5 bits of this register, the next most significant bit is
supplied by bit 7 of the Horizontal Sync End Register (CR05), and the 2 most significant
bits are supplied by bits 7 and 6 of the Extended Horizontal Blanking End Register (CR3C).
This 6-bit or 8-bit value should be programmed to be equal to the least significant 6 or 8
bits, respectively, of the result of adding the length of the blanking period in terms of
character clocks to the value specified in the Horizontal Blanking Start Register (CR02).
76543
210
A
Reserved
Display Enable Skew
Control
Horizontal Blanking End Bits 4-0
B
Reserved
Display Enable Skew
Control
Horizontal Blanking End Bits 4-0
Bit
6 5
Amount of Delay
0 0
no delay
0 1
delayed by 1 character clock
1 0
delayed by 2 character clocks
1 1
delayed by 3 character clocks